This invention relates generally to a control circuit for a three-phase AC motor and particularly to a solid state control system, comprising primarily digital logic circuitry, for controlling the operation of a three-phase submersible pump motor while at the same time protecting it against overload conditions.
In pumping fluid from an oil well, one conventional practice is to position a submersible three-phase electric motor and pump assembly in the well bore hole near the bottom thereof. A three-phase power supply of appropriate voltage (for example, 1500 volts per phase) couples to and drives the pump motor, which may have a rating of well over 200 horsepower. Since the average depth of an oil well is usually around 9,000 feet, severe heat and pressure may exist in the environment where the motor operates. It is important to shut the pump motor down in the presence of those conditions to prevent it from overheating. The motor should also be protected against overload or short circuit conditions which would pass more current through the motor than it could tolerate. Protection should also be provided against an underload or undercurrent condition, such as would occur when there is no fluid in the oil well, a condition known as "pump-off" . In addition to the motor shutdowns occasioned by undesirable conditions or environmental changes, it is usually desirable to turn the motor off automatically in response to certain physical conditions, such as when a retaining tank is filled up. Preferably, motor shutdown is delayed for a prescribed period to avoid unnecessary nuisance shutdowns resulting from transient effects or momentary conditions.
The present invention provides a novel overload protection system for such a pump motor. A unique digital logic arrangement is employed to vary the shutdown time delay inversely with the amount of overload so that greater overloads cause quicker shutdowns. Moreover, the overload protection system is of relatively inexpensive construction; requires very little power; is highly efficient, accurate and reliable; and can be contained within a very small space.